Expanding Our Understanding of Optical Nonlinearity for Information Processing and Electronics Applications

July 04, 2020 by Luke James

German researchers claim to have revealed a previously overlooked nonlinear mechanism that emerges from the light-induced tunneling of electrons inside dielectrics.

Before we can realize the full potential of ultrafast lightwave electronics and laser-based material processing, we must monitor and control the light-driven motion of electrons inside matter on a tight timescale of a single optical cycle. 

Physicists from the Max Born Institute in Berlin and the University of Rostock claim that they have revealed previously an overlooked nonlinear optical mechanism that emerges from the light-induced tunneling of electrons inside dielectrics.  


Nonlinear Optical Phenomena

Nonlinear optics describe the behavior of light in nonlinear media and our understanding of nonlinear optical phenomena. Such as self-focusing, solitary waves, and wave mixing comes from Kerr nonlinearity, which describes the nonlinear displacement of tightly bound electrons under the influence of an incident optical light field. 

When the intensity of this light field is sufficiently high, this picture changes as bound electrons eject from their ground state. At long wavelengths of the incident light area, this scenario is connected to the phenomenon of tunneling. In this quantum process, an electron performs a "classically forbidden" transit through a barrier formed by the combined action of the light force and the atomic potential. 

For several years now, the motion of electrons that have emerged at the "end of the tunnel" has been considered as an essential source for optical nonlinearity. That was, until now. 


Light emission from the current associated with light-induced electronic tunneling.

Light emission (blue) from the current associated with light-induced electronic tunneling inside a transparent dielectric material due to excitation with a strong optical field (red). Image credited to the University of Rostock

Changing the Picture

According to the research team, this is no longer the case thanks to their work. "In the new experiment on glass, we could show that the current associated with the quantum mechanical tunneling process itself creates an optical nonlinearity that surpasses the traditional Brunel mechanism," explains Dr. Alexandre Mermillod-Blondin from the Max Born Institute.

In their work, the team focused two ultrashort light pulses with different wavelengths and propagation directions onto a thin piece of glass before performing a time- and frequency-resolved analysis of the emerging light emission.

A theoretical analysis of the measurements enabled the identification of the mechanism responsible for this emission. "The analysis of the measured signals in terms of a quantity that we termed the effective nonlinearity was key to distinguishing the new ionization current mechanism from other possible mechanisms and demonstrating its dominance", said Professor Thomas Fennel. 


Dielectric Materials for Electronic Applications  

According to the researchers, future studies using this information and the novel metrology method they developed could help researchers resolve and steer strong-field ionization and avalanching in dielectric materials, which are an important class of thin-film electronic materials for microelectronics.

Applications of dielectrics include transistors, capacitors, and a swathe of other device components. If researchers can perfect dielectrics, it could be a boon for the miniaturization trend, which is currently driving endless examples of innovative and exciting research in the electrical and electronic engineering sectors.